1. Field of the Invention
The present invention relates to a printing head for a dot impact type printer, and more particularly, to a printing head having a distortion element which drives a print wire.
2. Description of the Related Art
As shown in FIG. 4, in a conventional type of printing head, a plurality of, for example, twenty four print units 2 for projecting print wires 1 forward (in the left in FIG. 4) are arranged to be attached to a circular support member 4 having a nose portion 3. Each of the print units 2 has a frame 5 made of a superinvar alloy. The frame 5 supports a piezoelectric element 6 and transmission mechanism 7. The plurality of print units 2 are circumferentially arranged in an annular shape at the back side of the support member 4. The back side is opposite to the side where a nose portion 3 is provided. The plurality of the print units 2 are attached as follows: a screw 9 is fastened to a nut member 8 fixed at the fore end 5a of the frame 5 through a fixing hole 4a formed in the periphery of the support member 4. The fore end 5a is the end where the transmission mechanism 7 is supported. On the other hand, a rear end 5b is the other end where the transmission mechanism 7 is supported. The tip end of the print wire 1 with the base end thereof fixed in the transmission mechanism 7 is adapted to project by a predetermined length through a through hole (not shown) formed at the tip end of the nose portion 3 in vertical arrangement.
With selective application of a voltage to the print unit 2 (i.e., the piezoelectric element 6), the extension of the piezoelectric element 6 can be amplified by the transmission mechanism 7 to be transmitted to the print wire 1. As a result, the print wire 1 at a given position can be projected, thus performing a print operation.
However, in the conventional construction as described above, the frame 5 is only screwed at the fore end 5a. Accordingly, the print unit 2 is swung on the screwed portion in the directions indicated by arrows X and Y in FIG. 4 together with the drive of the piezoelectric element 6. Therefore, there are problems inclusive of energy loss of the extension of the piezoelectric element 6, and deterioration in quality of print.
In order to solve the above explained problems, a printing head, in which the rear ends 5b of all frames 5 are bonded to a part of disk type member by an adhesive, with the disk type member fixed at the rear ends 5b of the frames 5 of the plurality of the print units 2 arranged in an annular shape, has been proposed. According to such printing heads, the plurality to print units 2 are supported by each other at the rear ends 5b of the frames 5 thereof, thereby preventing any swing of the frames 5 at the rear ends 5b thereof and any generation of vibration.
However, in the printing heads, in which all the print units 2 are fixed integrally by the adhesive as described above, when exchange of only one of the print units 2 is required in the case that one of the print wires 1 happens to be broken, the exchange work is very cumbersome.
Further, related design for fastening the fore end 5a and the rear end 5b of the frame 5 by screws is disclosed in the Japanese Laid-Open Patent Publication No. 63-312852.
However, there is a problem that the invar alloy constituting the frame 5 of the print unit 2 is difficult to machine. In addition, there is another problem that arises when both ends 5a, 5b of the frame 5 are fastened by screws. The frame 5 is drawn lengthwise due to a dimensional differences in the lengths of the frames arising from manufacturing tolerances, so that the energy loss of extension of the piezoelectric element 6 is caused. Therefore, the quality of the printing is deteriorated.